fix: genirq: Restrict export of irq_to_desc() (v5.11)

fix: genirq: Restrict export of irq_to_desc() (v5.11)

See upstream commit:

  commit 64a1b95bb9fe3ec76e1a2cd803eff06389341ae4
  Author: Thomas Gleixner <tglx@linutronix.de>
  Date:   Thu Dec 10 20:26:06 2020 +0100

    genirq: Restrict export of irq_to_desc()

    No more (ab)use in drivers finally. There is still the modular build of
    PPC/KVM which needs it, so restrict it to this case which still makes it
    unavailable for most drivers.

Link: https://lore.kernel.org/r/20201210194045.551428291@linutronix.de
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I9c092187fbc242e5e47a5c248e76ef658ff9f562